Thoughts please?

A friend has a 2004 Buick Rainer which currently has about 70,000 miles on
it.  Perfectly smooth car until about seven or eight thousand miles ago.
Then it began to show to me what I felt was 'driveshaft' vibration.  At
least the vibration was definitely related to shaft rotation speed.  The
vehicle also had developed tire vibration as well.  Which, in fact, masked
the driveshaft speed vibration enough so that I really didn't pick up on it
as early as I should have.  OK, new set of Michelin tires complete and with
a very good balance job as well as wheel check.  Interestingly, one of the
old tires, in the rear, actually had a double 'hump' out of round issue,
each hump being 180 degrees away so that it double bumped per tire
rotation!  Interesting.

A few thousnad miles before this issue, this SUV also began to show some
irregular overnight rear end drop.  It was an air bag rear suspension
model, also only rear end drive and with limited slip differential as well.
 That got worse and a tear down revealed air bag leaks in this as-built
trailer towing model so equipped.  This issue was looked at AFTER the
original complete new tire installation.  And the driveshaft speed related
vibration was definitely there following the new tire installation as was
done.  It was particularly noticeable up at between 60MPH and about 78MPH,
but also very defintely there between just above 30MPH and 40MPH as well. 
However, it did show up enough for an aware person to notice it between
40MPH and 60MPH as well.

In that the air bag issue 'popped up' more severely at that point, this
matter was the next thing serviced.  With no need for trailer towing to be
used in the future, as well as advice that replacing air bags weould just
present more of the same failures in the future, the decision was made to
replace the air bags with steel springs.  Which was done very well with NO
possibility of any driveshaft bending or damage.  In that the shocks for
the air bag design were stiffer than what was told to be for the steel
spring design version which is also used for this vehicle, new matching
shocks were installed as well.  Following that work, the vehicle level
matched exactly the same at rest as it had been with the air bags.

It was a little stiffer along the roadway, but seemed to be just fine.
However, now the 'driveshaft' vibration seemed to be stronger and, as the
next couple thousand miles rolled up, got MUCH worse.  Especially down
right over 30MPH and up just above 60MPH.  In fact at 71MPH to about 76MPH
it was REALLY bad and just getting worse and worse.

The complete tire balance and roundness as well as wheel roundness was
carefully checked at two different places.  Perfect,  At the next real
service shop, they had it up on a lift and could spin it up and very
definitely feel it.  They yanked the shaft, lowered the vehicle and spun up
the rear end with a friction wheel balancer on first one then the other
wheel.  No vibration.  However, the technician did find that the front
U-Joint was VERY badly worn out.  He replaced both U-Joints and even that
done, the vibration was still there, though somewhat softer.  OK, this is
an alumninum shaft vehicle.  There is no shop here in the Bryan-College
Station, Texas, area which can handle aluminum shaft balance issues, so he
got a complete other exact model shaft from a local wrecking yard and put
it in the vehicle.  Same problem!  Telling us this had to be transmission
problems, the vehicle went to a local transmission shop that I have worked
with for over thirty years.

They serviced the transmission, found almost NO evidence of steel or brass
filings in the fluid, nor could they find and evidence of a thing wrong in
the transmission.  As well, they checked the motor mounts and as well
agreed with me that there was no evidence of vibration from the
engine/transmission and even while driving, there was absolutely no change
in the vibration from shifting it into neutral during the vibration, nor
from brake tapping to pop the torque converter out of direct into fluid
mode, nor from even downshifting into the next lower gear during driving. 
Now it gets interesting.  We've been together for decades as I mentioned. 
So they got a tow truck.  They took the shaft out and lifted the front end,
then towed it down the highway up to at least 80MPH while riding in it
studying for vibration!  Absolutely no vibration at all.  Smooth as silk
ride.

OK, there is a really decent driveshaft service operation here that has
been here for decades as well.  Though they can't handle aluminum shaft
work, we all decided that they should build a steel shaft for this less
than six foot long driveshaft and balance it.  OK, it went into the
vehicle.  Much smoother!  But still there at just over 30MPH and still
there at 64MPH and up at 71MPH to 78MPH, it still was totally wrong such
that it could not normally be driven and would chew things up if just left
that way.  This group does not do rear-end service work and there is
another very capable shop here that does.  The vehicle went out there and
the shop owner said it was definitely shaft vibration.  But he also noted
that the ONLY way you could handle any of these aluminum shaft vehicles,
even if you were to use a steel shaft in them, was to do the shaft
balancing with a LASER balancing lathe!  And there was only one shop he had
found down in Houston to which he had sent or sent the complete vehicle for
55 different shaft jobs which was the only place he had found that could
handle this!  Yes, the local shop did fine with new steel shafts and
blancing, but with the aluminum shaft vehicles this was the only answer he
had ever found that worked.

OK, I took the vehicle to Houston.  They determined that the complete
balance job on the new steel shaft was absolutely wrong, as well as that
the new U-joints in this shaft were not good enough either!  As well this
shaft as built had a brand new GM supplied front yoke in it which they said
was fine.  But ... as laser balanced there in Houston, although the vehicle
was MUCH better yet, there was still a tiny bit of vibration down at 30MPH
which the average person would never have noticed.  As well, there was
still the same 60MPH+ vibration, and still the really bad 71MPH+ vibration.
 At that point the crew there recalled they had a brand new aluminum GM
shaft somebody had bought but never picked up,so they crammed the new yoke,
new U-Joints into it and laser balanced it and we tested it.  It was even
the same part number and source!

No vibration at all any more in the 30MPH range,  Very little at all in the
60MPH range though a TINY bit could be felt at 69MPH and up until 74MPH. 
But from 74MPH to about 78MPH the bad vibration was still there, though not
at all as jaw shaking as it had been.  So told me there is no question but
the transmission is clean.  This definitely being felt at the rear,  But
there is no pinion seal leakage and though there is a little more
rotational slop than expected, there is no felt pinion irregulartity.  The
only thing that looks like it can be done next will be to yank the third
member and attempt to trace this to pinion problems, one way or another. 
And there is no rear suspension angularity problems or damage.  Now I am
familiar with the very rare issue with a rear pinion bearing problem which
does not result in pinion seal issues.  But in that there is no really
noteable pinion rotation irregularity seen, even if this is a rear pinion
bearing crack or whatever problem, I wonder?

I'm very close to a Texas Aggie graduate friend who is a lot better at this
mechanical stuff than I am.  He was a double degree guy in both mechanical
and electrical engineering.  The design guy later for the whole hydraulic
motor floating anchor and drilling rig precision location centering for sea
drilling rigs.  As well as, among lots of other stuff,he actually was in
the designer for the Howard Hughs hydraulic stuff that carried up the
Russian recovered vessel in the Hunt for Red October. He and I agree.  This
has to be a reasonance issue as well.  But that said, how can this be just
a pinion bearing or third member problem? What is going on here?  Where is
the reasonance and why?

Thoughts?

Mike Luther N117C at 1:117/100


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